Chiral Dienes: From Ligands to FLP Catalysts

Asymmetric catalysis by chiral FLPs: A computational mini-review

Steric hindrance in Lewis acid (LA) and Lewis base (LB) obstruct the Lewis acid-base adduct formation, and the pair was termed as frustrated Lewis pair (FLP). In the past 16 years, the field of enantioselective catalysis by chiral FLPs has been slowly growing. It was shown that chiral LAs are significant as they are involved in the hydrogen transfer (HT) step to the imine, resulting in enantioselectivity. After H2 activation, the borohydride can exist in a number of plausible conformations and their stability is governed by the presence of noncovalent interaction through C-H····π and π····π interactions. However, LBs are not ideal for asymmetric induction as they compete with the imine substrate as a counter LB. Further, the proton transfer from chiral LB to the imine does not induce any chirality as chirality develops in the HT step. However, intramolecular FLPs with chiral scaffold are very efficient as they possess an optimum distance between LA and LB, which facilitates the H2 activation but precludes the adduct formation of the small molecules substrate with the LA component. This mini-review summarizes computational investigation involving chiral LA and LB, and discusses intramolecular FLPs in the enantioselective catalysis.

Asymmetric Transfer Hydrogenation of 2-Substituted Quinoxalines with Regenerable Dihydrophenanthridine

The asymmetric hydrogenation of quinoxalines represents one of the most efficient approaches for the synthesis of optically active tetrahyroquinoxalines. In this paper, we demonstrate a metal-free asymmetric transfer hydrogenation of 2-substituted quinoxalines with regenerable dihydrophenanthridine under H2 using a combination of chiral phosphoric acid and achiral borane as catalysts. A wide range of optically active 2-substituted tetrahydroquinoxalines were produced in high yields with ≤98% ee.

Synthesis of Tetrasubstituted Enamines through Borane-Catalyzed Hydrogenations

This paper describes a B(C6F5)3-catalyzed hydrogenation of β-substituted α,β-unsaturated imines by using as low as 0.2 mol % catalyst. A variety of tetrasubstituted enamines were afforded in 95-99% yields. It provides an efficient and facile way without the need for column chromatography purification.

Stereoselective Hydrosilylation of 1,2-Diketones Catalyzed by Chiral Frustrated Lewis Pairs

The asymmetric reduction of 1,2-diketones for the synthesis of optically active 1,2-diols, especially 1,2-anti-diols, remains a formidable challenge. In this paper, we describe the first highly stereoselective hydrosilylation of unsymmetrical vicinal diketones with PhSiH3 by using a chiral frustrated Lewis pair (FLP) catalyst, giving a variety of 1,2-diaryl-1,2-anti-diols in high yields with excellent d.r. values and up to 97 % ee. The chiral FLP catalyst exhibits the ability to control regio-, diastereo- and enantioselectivites concurrently.

Asymmetric Hydrogenations of Acyclic α,β-Unsaturated Ketones with Chiral Frustrated Lewis Pairs (FLPs)

In this paper, we demonstrate a metal-free asymmetric hydrogenation of acyclic α,β-unsaturated ketones under the catalysis of a frustrated Lewis pair (FLP) comprising chiral oxazoline and achiral borane. A wide range of optically active α-substituted ketones were furnished in high yields with 26-85% ee's.

Asymmetric catalysis with FLPs

Chiral catalysts play a crucial role in the realm of asymmetric catalysis. Since their breakthrough discovery in 2006, chiral frustrated Lewis pairs (FLPs) have risen as a novel catalyst category for a broad range of metal-free asymmetric reactions. This review provides an overview of the remarkable progress made in this field over the past 15 years. The design and synthesis of chiral FLPs and their applications in hydrogenation, hydrosilylation, transfer hydrogenation, and various other reactions are summarized and highlighted.

Asymmetric Reduction of Quinolines: A Competition between Enantioselective Transfer Hydrogenation and Racemic Borane Catalysis

A chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of quinolines with regenerable dihydrophenanthridine derived by a borane-catalyzed hydrogenation of phenanthridine under H₂ has been successfully realized. Despite the competition of a racemic hydrogenation pathway, a variety of tetrahydroquinolines were furnished in high yields with up to 91% ee.

Chiral Diene Ligands in Asymmetric Catalysis

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.